chore: set up 2.12.x branch (#4276)

Author: mutianf

.github/release-please.yml

@@ -56,3 +56,12 @@ branches:
         bigtable-client-core-parent/bigtable-hbase/src/main/java/com/google/cloud/bigtable/hbase/BigtableHBaseVersion.java
     handleGHRelease: true
     branch: 2.9.x
+  - releaseType: java-backport
+    bumpMinorPreMajor: true
+    extraFiles:
+      - >-
+        bigtable-client-core-parent/bigtable-client-core/src/main/java/com/google/cloud/bigtable/config/BigtableVersionInfo.java
+      - >-
+        bigtable-client-core-parent/bigtable-hbase/src/main/java/com/google/cloud/bigtable/hbase/BigtableHBaseVersion.java
+    handleGHRelease: true
+    branch: 2.12.x

.github/sync-repo-settings.yaml

@@ -100,6 +100,20 @@ branchProtectionRules:
       - OwlBot Post Processor
     requiredApprovingReviewCount: 1
     requiresCodeOwnerReviews: true
+  - pattern: 2.12.x
+    isAdminEnforced: true
+    requiredStatusCheckContexts:
+      - 'Kokoro - Test: Binary Compatibility'
+      - 'Kokoro - Test: Code Format'
+      - 'Kokoro - Test: Dependencies'
+      - 'Kokoro - Test: Integration'
+      - 'Kokoro - Test: Java 11'
+      - 'Kokoro - Test: Java 8'
+      - 'Kokoro - Test: Beam Integration'
+      - cla/google
+      - OwlBot Post Processor
+    requiredApprovingReviewCount: 1
+    requiresCodeOwnerReviews: true
 permissionRules:
   - team: yoshi-java-admins
     permission: admin